1. Field of the Invention
The present invention relates to an automotive brake control system which carries out what we call ABS control (skid control) for controlling a brake fluid pressure to prevent wheels from locking during braking. More particularly, it relates to a technology for processing performed when abnormality occurs due to noise superimposed on the output of wheel speed sensor or external turbulence caused by poor installation.
2. Description of Related Art
For the brake control system for carrying out ABS control, a pseudo vehicle body speed is estimated from the wheel speed of each wheel, and the brake fluid pressure (wheel-brake cylinder pressure) is automatically reduced, held, and built up based on this pseudo vehicle body speed and the wheel speed of each wheel to prevent wheels from locking during braking. Thereby, the steering ability (steering control) and running stability (vehicle stability) are secured, and at the same time the braking distance is shortened.
For such an automotive brake control system, the detection accuracy of wheel speed is important in executing the aforementioned ABS control, and if an abnormality of a wheel speed sensor occurs, it is difficult to execute the normal ABS control. Therefore, a brake control system has been proposed which carries out control corresponding to a detected abnormality if abnormality of wheel speed sensor occurs.
As such a conventional brake control system, a brake controller disclosed in Unexamined Japanese Patent Publication No. 10-147232 has been known.
In the system disclosed in the Japanese Patent Provisional Publication No. 10-147232, the controller has an abnormality detecting means for detecting abnormality of each wheel, and for calculating estimated vehicle body speed from each wheel speed of wheels other than the wheel in which abnormality is detected with respect to estimated vehicle body speed calculating means at the time of abnormality detection. This abnormality detecting means is configured so that if the wheel speed calculated in this control cycle of the same wheel increases as compared with the wheel speed calculated in the previous control cycle exceeding a set value xcex1, and increases as compared with the estimated vehicle body speed (pseudo vehicle body speed) exceeding a set value xcex2, the wheel speed calculated in this control cycle on this wheel is judged to be abnormal. Further, it is configured so that when abnormality is found, the estimated vehicle body speed is calculated from the wheel speeds excluding a wheel on which abnormality is found.
In this conventional system, therefore, in the case where noise is superimposed on the output of wheel speed sensor, and the output is increased, when the wheel speed is higher than the previous value by xcex1 and more, and higher than the estimated vehicle body speed by xcex2 and more, abnormality is found, so that this wheel speed is not used in estimating the vehicle body speed, and the estimated vehicle body speed is not higher than the actual vehicle body speed. Thus, there is a reduced possibility that skid control is undesirably executed and the pressure reduction is undesirably executed. Additionally, it is possible to prevent the system from erroneously determining whether or not the abnormality is present when the wheel speed is rapidly recovered from a skidding state, thereby preventing the skid control or pressure-reduction mode from being executed wastefully.
In the system disclosed in the Japanese Patent Provisional Publication No. 10-147232, when abnormality such that noise is superimposed on the wheel speed sensor occurs, the vehicle body speed is estimated excluding a wheel on which abnormality occurs, by which the control accuracy can be improved. However, since the judgment of abnormality of wheel speed sensor is made by a comparison of the output value of wheel speed sensor and pseudo vehicle body speed and a comparison with the previous output value, problems described below may be caused.
In generating or producing the pseudo vehicle body speed, in the first cycle of control in which ABS control (skid control) is initiated, each wheel tends to be locked, so that an accurate wheel speed cannot be obtained. Therefore, apart from an expensive control system having a longitudinal acceleration sensor, in an inexpensive control system having no longitudinal acceleration sensor, calculation is made by subtracting a fixed value from the previous pseudo vehicle body speed.
Thereupon, in particular, on a road with a low coefficient of friction (hereinafter referred to as low-xcexc road), the deceleration of vehicle body speed is low, so that a pseudo vehicle body speed Vi determined by subtracting a fixed value in this manner is sometimes lower than the actual vehicle body speed. Contrarily, when ABS control for avoiding the locking of wheel is carried out, the wheel speed is once increased to a value substantially equal to the actual vehicle body speed by processing of pressure reduction. As a result, the wheel speed becomes higher than the pseudo vehicle body speed. When this speed exceeds the aforementioned value xcex2, abnormality may be found mistakenly.
When the driving wheel slips and thus the wheel speed of the driving wheel becomes higher than the pseudo vehicle body speed, likewise, abnormality may be found mistakenly.
The present invention has been made to solve above problems, and accordingly an object thereof is to prevent mistaken judgment on a low-xcexc road and mistaken judgment due to slip of a driving wheel, thereby improving the abnormality judgment accuracy and upgrading the control quality in a brake control system capable of executing ABS control, in which when abnormality such as superimposition of noise on the output of a wheel speed sensor occurs, a pseudo vehicle body speed is generated on the basis of wheel speeds excluding an abnormal wheel.
In order to accomplish the aforementioned and other objects of the present invention, an automotive brake control system comprises a hydraulic brake unit connected to each individual wheel cylinder, for individually controlling a wheel-cylinder pressure to the individual wheel cylinder, wheel speed sensors detecting wheel speeds of each of road wheels, and a control unit being configured to be electrically connected to the wheel-speed sensors and the hydraulic brake unit, for processing wheel-speed sensor signals from the wheel-speed sensors and generating a command signal to the hydraulic brake unit to prevent wheel lock-up, the control unit comprising a pseudo vehicle body speed generating section generating a pseudo vehicle body speed based on the wheel-speed sensor signals, a skid control section executing skid control based on a difference between the pseudo vehicle body speed and each of the wheel-speed sensor signals, to prevent wheel lock-up during braking by momentarily reducing the wheel-cylinder pressure through the hydraulic brake unit, and an abnormality judgment section determining whether an abnormal wheel-speed sensor signal is included in the wheel-speed sensor signals from the wheel-speed sensors, and cooperating with the pseudo vehicle body speed generating section for generating the pseudo vehicle body speed based on the wheel-speed sensor signals except the abnormal wheel-speed sensor signal when the abnormality judgment section determines that the abnormal wheel-speed sensor signal is present, the abnormality judgment section including a pressure-reduction time counter which measures a pressure-reduction time from a time when a pressure-reduction operating mode is initiated due to a rise in the pseudo vehicle speed during the skid control, and an abnormality decision circuitry which determines that the abnormal wheel-speed sensor signal is present when the pressure-reduction time exceeds a predetermined time period.